Fluid flow control equipment, such as pumps, compressors, agitators, and valves are widely used in industries such as utilities, refineries, chemical and petrochemical. These equipment generally serve a fluid flow control function, and each typically includes a rotary or reciprocating shaft. The shaft can be motor driven or manually operated. The shaft rotates in order to control fluid flow through the equipment. For example, the shaft of a rotary pump operatively connects to a motor on the exterior of a pump casing to an impeller or blade on the interior. The motor rotates the shaft, which in turn rotates the impeller. In a valve, the shaft connects to a gate on the interior of the casing for controlling flow of fluid through the valve. Thus, there are at least three openings in the pump (or valve) casing: a first fluid opening for an inlet pipe, and second fluid opening for an outlet pipe and an opening for the shaft.
The two fluid openings for the inlet and outlet pipes are sealed conventionally. The shaft however passes through a recessed area within the pump (or valve) known as the "stuffing box". The term stuffing box is derived from the method employed to prevent fluid from leaking through this opening in the casing for the shaft. The fluid is contained within the pump by stuffing or packing a material around the shaft to seal the opening. The packing material in the stuffing box thus functions to protect the fluid flow equipment against leakage where the rotating or reciprocating shaft or valve stem extends through the casing.
For a number of reasons, rotating and reciprocating shafts are difficult to seal. In operation, the shaft is capable of both radial and axial displacement. Radial displacement typically results from manufacturing inaccuracies or could result from wear caused by metal-based packings rubbing against the shaft during operation. Axial displacement results from different thermal expansions produced through normal operation of the shaft. In addition, conditions in the stuffing box are constantly changing. For example, shaft speeds may vary. The packing may be required to withstand high temperatures and pressures followed by low temperatures and pressures. The surfaces of the shaft in the stuffing box are often pitted and rough, causing excessive and uneven wear of the packing material. Friction between the shaft and the packing produces heat. Excessive heat can cause packing to harden and loose resiliency thereby creating spaces and gaps where leakage can occur. Further, environmental concerns have lead to increased awareness of emissions of volatile fluids from pumps and valves. Government regulatory agencies and government legislation have mandated reduced emissions from pumps and valves used in industry. In particular, the petrochemical industry faces significant reductions in allowable emissions. The 1990 Clean Air Act has mandated reductions in fusitive emissions as part of the efforts to improve air quality. Fugitive emissions are fumes and gases that escape to the atmosphere from valves, pumps and pipes in processing plants such as refineries. The reduced levels are called "zero emission" because the permitted levels are 500 parts per million and less, depending on the severity of the ambient air quality. The trend is towards further reductions of permissible levels of emissions, even to 10 or fewer parts per million. Many industrial plants must install improved packing to meet the air quality requirements that fugitive emissions be reduced significantly.
Various types of packing for a stuffing box are noted in the prior art. These packings include soft packing, metallic packing and graphite packing. Soft packing generally is made from fabric, hemp, or rubber fibers woven into strands and formed into a braided length. Metallic packing incorporates flexible metallic strands or foils in a soft packing core. Metallic packing has several advantages over soft packing. These advantages include improved maintenance for the fluid flow equipment. Metallic packing is easier to pick and remove from the stuffing box as a unified piece than is soft packing. Further, the metallic packing provides resiliency for conforming the packing to the shaft. Expanded graphite in the form of a solid annulus or ring also provides a seal for a stuffing box. Graphite provides a seal with high temperature, high pressure capability.
Recent developments in packing materials have been made in response to the increased environmental concerns, discussed above. One packing provides a resilient flexible core of longitudinally braided yarns having an exterior graphite skin. This skin is spirally wound in an overlapping manner about the core. Such spirally wound packing, however, was not satisfactory as the overlapping edges provided leak paths through the packing for fluids. In addition, the spirally wrapped packing material could not be satisfactorily formed into a spiral coil for bulk distribution. Instead, this packing was supplied as a preformed ring having an inner diameter and an outer diameter sized to fit a particular stuffing box. Soft packing typically was provided in bulk form as a spiral coil. The appropriate length of packing was cut from a coil of packing having the cross-section width for the particular fluid flow equipment. Maintenance inventories therefore held coils of packing in the relatively few standard cross-sectional widths. Inventories of pre-formed rings however were quite large, as there are no standard sizes of inner and outer diameters for valves and pumps. For example, a five-inch valve made by one manufacturer may have a two-inch stem in a three inch diameter stuffing box. A five-inch valve made by another manufacturer may have a three inch stem in a four inch stuffing box. Each requires a one half inch cross-sectional packing, but the ring diameters are different.
Another known packing provides a flexible core of longitudinally braided yarns with a graphite skin of expanded graphite foil wrapped longitudinally about the core. While this packing has met with satisfactory results in many sealing situations, and especially for petrochemical sealing to meet the reduced emission requirements, the packing has drawbacks discussed below.
This packing is manufactured by dipping a length of fiber mesh into a mixture of graphite powder, binder and solvent. The mixture dries resulting in a flexible core possessing the beneficial maintenance features of the wire mesh core and the graphite lubrication for high temperature, high pressure sealing. The core is then longitudinally wrapped with a skin element made of expanded graphite having a layer of adhesive. The resulting sealing material is passed through a coiling device that densifies the material to the desired degree as well as spiralling the packing into a coil for bulk handling.
While the packing has met a need in the industry and has successfully sealed valves under extreme conditions, this packing has several drawbacks. The packing can only be manufactured by hand in relatively short lengths, typically of about ten feet or less. A sheet of expanded graphite is hand wrapped longitudinally around the flexible core to form the skin sheet. The sheet entraps air as it is folded around the core. The density of the packing material thereby varies. Hand wrapping of the graphite skin also is time consuming. Hand wrapping also lacks uniformity of the wrap around the core. While the packing does not have the plurality of seams, such as found in the spiral wrapped packing, the single seam still provides an opportunity for gaps to form in the seal in the stuffing box. It is typically recommended that the seam be positioned against the wall of the stuffing box instead of against the shaft. Further, the adhesive can contribute about 20% of the packing by weight. The adhesive increases the bulk of the packing and limits the temperature and pressure at which the packing can be used. Adhesive exuded from the packing under pressure can cause the shaft to freeze by adhering the shaft to the packing and the stuffing box.
Accordingly, there is a need in the art for a packing material having a resilient core and a uniform non-seamed surface, and a method of manufacturing such packing.